Bone fixation systems can include external bone fixation systems that are typically attached to two or more bone segments so as to stabilize the bone segments and promote healing. The external bone fixation systems can be applied to treat comminuted, intra-articular and/or unstable fractures. Thus, the bone segments can be fractured segments of a bone, or can alternatively be two different bones, for instance vertebrae, that are to be stabilized relative to each other. Typical external fixation systems can include a plurality of bone anchors that are configured to be driven through the dermal surface and into respective bone segments. For instance, the bone anchors typically are configured as bone screws, such as Schanz screws, that have a length sufficient such that they extend out from the epidermis when anchored in the respective bone segments.
External fixation systems can further include at least one support rod, and at least one set of clamps that are configured to be secured to both the rod and the bone anchors, thereby securing the bone anchors relative to the rod, and supporting each the bone fixation members relative to the other bone fixation members secured to the rod. External fixation system can further include clamps that are configured to be secured to a pair of rods, so as to secure each of the pair of rods to the other. External fixation systems further commonly include coupling members that are configured to attach to both the support rod and one or more of the Schanz screws, such that the Schanz screws, and thus the bone segments, are supported by the rod in fixation with the respective bone segments.
Conventional support rods, clamps, and bone fixation members are typically made from an electrically and thermally conductive material stainless steel, titanium, alloys thereof, or any suitable alternative metal. Though support rods can be made from a non-ferromagnetic material, such as such as aluminum or carbon, the external fixation system in combination with the soft tissue into which the external fixation system is implanted can define a closed electrical loop. As a result, when the external fixation system is subjected to the magnetic fields (typically having a strength between and including 1.5 Tesla and 3.0 Tesla, but can range up to and including 8.0 Tesla) and radio frequency pulses of a magnetic resonance imaging (MRI) system, electrical current can be induced in the closed electrical loop. The current flow can cause the temperature of the thermally conductive Schanz screws to rise substantially inside the patient's body, resulting in pain and damage to the tissue.
External fixation systems have been proposed that are said to reduce or prevent current when the implanted exposed to the RF field of an MRI. For instance, U.S. Pat. No. 7,527,626 discloses that the rod and/or clamps can include a carbon core and a polymeric insulation sheath that is applied onto the carbon core through resin transfer molding. The patent recognizes that the size of the carbon core of the rod “must” be reduced so that once the sheath is applied to it, the resulting product has the same size as rods typically used in external fixation systems. Accordingly, the core is made of a higher modulus carbon fiber. Thus, the sheath can add cost and complexity to the manufacture of the external fixation system